Various properties of paper products, including strength, opacity, smoothness, porosity, dimensional stability, pore size distribution, linting propensity, density, stiffness, formation and compressibility are primarily are to a large extent dependent on the bonds which exist between the cellulosic fibres in the paper. The bonding capability of these fibers is enhanced by the mechanical beating or refining step(s) of the papermaking process, during which the fibers are made more flexible and the available surface area is increased.
The strength of paper products is a property having three categories, referred to as dry strength, wet strength or rewetted strength, and wet web strength. Dry strength is the tensile strength exhibited by the dry paper sheet, typically conditioned under uniform humidity and room temperature conditions prior to testing. Wet strength, or rewetted strength, is the tensile strength exhibited by a paper sheet that has been fully dried and then rewetted with water prior to testing. Wet web strength is strength of a cellulosic fiber mat prior to drying to a paper product.
Strength resins are polymers generally added at the wet end of the papermaking process to the cellulosic slurry, prior to the formation of the paper mat or sheet, to improve the strength characteristics of the paper product. Strength resins are generally believed to work by supplementing the number of interfiber bonds.
Dry strength additives are used to increase the dry strength of various paper products including paper, paperboard, tissues and others. Dry strength additives are particularly useful in the manufacture of paper products from recycled fibers, as recycling is known to have a weakening effect on the resulting paper. In addition, dry strength additives should reduce the amount of refining required to achieve a given dry strength for a given pulp, and the corresponding energy consumption required for refining and should not adversely affect the drainage rate of the cellulose web on the papermaking machine.
Various polyacrylamides have been used as wet-end additives to increase dry strength of paper products. The polyacrylamides are particularly useful as dry strength additives as they contain primarily amide groups which can form hydrogen bonds so that the interfiber bonds in the paper sheet increase without the increase in wet strength caused by covalent crosslinks. The polyacrylamides are generally also water-soluble which leads to a uniform distribution in the stock and a uniform adsorption on the surfaces of the fibers. See Paper Chemistry, An Introduction, D. Eklund and T. Lindstrom, DT Paper Science Publications, Grankulla, Finland (1991).
The use of certain cationic copolymers of acrylamide and .alpha.,.beta.-unsaturated quarternary ammonium compounds as dry strength additives is disclosed in U.S. Pat. No. 2,884,057.
A polyelectrolyte complex comprising at least one high molecular weight, low charge, water soluble cationic polymer and at least one anionic polymer for increasing the dry strength of paper is disclosed in U.S. Pat. No. 5,338,406. Copolymers of acrylamide and diallyldimethylammonium chloride or methacryloyloxyethyltrimethylammonium chloride are recited as preferred cationic polymers.
A papermaking process that utilizes mixtures of wet strength agents and dry strength agents, including cationic acrylamide-diallyldimethylammonium halide copolymers, to produce paper having increased wet strength without compromising dry strength is disclosed in PCT/US98/01980.
However, the need still exists for a sole treatment agent for increasing the dry strength of paper products.